Terahertz (THz) systems and technology have become important for a wide variety of applications in analysis, diagnostics, and communications. A large number of significant technical innovations over the last decade have made the terahertz frequency range (0.1 to 10 THz) increasingly accessible for applications in science and industry. However, difficulty in generating, manipulating, and detecting terahertz radiation continues to plague most applications. Source powers (from sources that are readily attainable) tend to be low. Focusing optics are costly, difficult to align, and have high loss. Detectors are expensive, inefficient (particularly at high frequencies), and usually require alignment with an optical pump source. In combination, these factors restrict terahertz applications to exploratory and scientific investigation, rather than to what could become large markets for relatively high-volume applications.
One of the most common applications for THz technology is spectroscopy. In spectroscopy, the frequency dependence of transmission of electromagnetic radiation through a sample reveals a unique fingerprint of the substances being studied. In the THz frequency range, this fingerprint results from unique rotational and vibrational energy states associated with complex molecules, providing information related to composition and conformal state. While other approaches (e.g. Raman), offer views into molecular form and function, THz spectroscopy provides a unique and complementary view. As a result, an easy to use and cost-effective THz spectrometer will stimulate environmental, medical, material science, and other applications.
Both time-domain spectrometers (TDS) and frequency-domain spectrometers (FDS) are well established. At the heart of each, as shown by the dotted lines in FIG. 1, is a THz generation-analysis-detection (TGAD) module 100. In a typical TDS, a short laser pulse (50 fs) is split and focused on separate transmit and receive photomixers (PM) 110. The terms photomixer and photoconductive switch (PCS) are generally used in reference to FDS and TDS operation, respectively. Within this document and in the claims, it is understood that the terms PM and PCS are applicable to both modes of operation. The transmit PM is biased such that the change in conductivity induced by absorption of the short optical pulse (TDS) or high-frequency modulation component (FDS) creates a corresponding current transient that drives a small dipole antenna, radiating THz frequencies that are captured by a silicon lens 120 to produce a diverging THz beam 130. A Teflon lens, or alternatively a metallic focusing reflective optical element, 140 then creates a collimated THz beam region 150 in which a sample can be situated. After passing through a sample, the THz beam is delivered to the receive PM where it is sampled by the short laser pulse (TDS) to produce a small current as measured typically with a lock-in-amplifier. In order to accurately sample the received THz signal, the receiver PM must possess an extremely short carrier lifetime, such that sampling signal is approximately a delta function or impulse in time. For this reason, low-temperature grown (LT) GaAs is usually used for the PMs, in which defects within the crystal lattice provide rapid carrier recombination. The sampling time is adjusted using a variable optical delay, in this case introduced by varying a retro-reflector. FDS operate in the same manner except that instead of using a short laser pulse, two continuous-wave lasers separated in optical frequency by the THz frequency are combined to produce the THz pump intensity variations. THz pump intensity variations could also be generated, in principle, using a broadband optical source like a super-luminescent diode or amplified spontaneous emission source.
Other methods of system operation have been demonstrated. For example, it is not necessary to use PCS devices at the detector. Other nonlinear optic detection methods, in particular using electro-optic crystals have been studied. For example, using the Pockel's effect, a nonlinear crystal with high electro-optical coefficient, such as ZnTe, undergoes induced birefringence when a THz beam propagates inside the crystal. This changes the polarization of a co-propagating probe beam from circular to elliptical, changes that can be detected using a Wollaston beam splitter and a differential photo detector pair.
Many applications for THz analysis have been defined, including measurement of gas-phase samples, liquids and solids. Examples include non-invasive diagnostics of disease by detecting volatile compounds in breath, such as ethanol (C2H5OH) for law enforcement, hydrogen for carbohydrate metabolism, nitric oxide (NO) for asthma, carbon monoxide (CO) for neonate jaundice, 13CO2 for H. pylori infection (related to stomach cancer and normally asymptomatic), and branched hydrocarbons for heart transplant rejection. Additionally, increased breath ammonia (NH3) is found to relate to kidney and liver dysfunction, breath acetone is higher in diabetes, and the level of aldehydes such as methanol (CH2O) can be used to screen lung and breast cancers. Currently applied analytical instruments include mass and mid-IR spectrometers. Although these are large and expensive, both are in widespread use, supporting diverse instrumentation and diagnostic industries. THz spectrometers can record fast processes, opening up a unique potential for real-time analysis of exhaled air.
In security, several applications exist in the area of explosive detection, where flames, plumes, and explosive vapour are of great interest. Collective motions in molecules found in common explosives correspond to features in THz spectra that can be used for unique identification. There have been extensive studies on the THz spectroscopy of explosives like DNT, RDX, HMX, TNT, and PETN10.
Environmental applications include measurement of rotational transitions of light polar molecules and low-frequency vibrational modes of large molecular systems, both of which can be probed by THz spectroscopy, opening applications in sensing atmospheric pollutants and detecting airborne chemicals. Atmospheric pollutants like hydrogen sulphide (H2S), OCS, formaldehyde (H2CO), and ammonia (NH3) possess intense THz transitions. Volatile organic compounds (VOCs) are of high interest in manufacturing and oil and gas industries, and are potentially detectable in real time using THz technology.
Finally, large numbers of applications have been considered within the laboratory, including studying the absorption and dispersion of compounds, real-time trace gas detection, and the analysis of chemical compositions.
THz generation-analysis-detection (TGAD) modules used in existing THz spectrometers (FIG. 1) use photomixers (PM) or photoconductive switches 110 to generate and receive THz radiation. FIG. 2 illustrates a detailed view of the PCS in the configuration of a typical photoconductive antenna (PCA) 210. The PCA comprises a PCS 211 and patterned metallic circuit 212, and is generally fabricated on a direct-band-gap semiconductor (low-temperature GaAs or InGaAs) substrate 213. The PCS utilizes photoconductivity of a semiconductor to generate free electrons and holes or photocurrent under intense laser illumination. The fast rise of the photocurrent resulting from the absorption of rapidly varying optical pump intensity, together with a short carrier lifetime of these photo-generated carriers, result in conversion of ultra-high speed pump intensity modulation into a corresponding THz modulation of the PCS conductance. At a transmitter, a bias voltage (e.g., 10 V) is applied across a small, typically a few μm, gap 211 in the conductive cap layer. The current through this gap is modulated by the modulation of the PCS conductance and the modulated current is radiated by the short dipole antenna (214). In the absence of light the semiconductor has a very low conductivity. This reduces the dark current and heat generated by the dark current. Lower dark conductivity allows more charges to build up at greater bias voltages while maintaining thermal stability. Upon the arrival of a femtosecond optical pulse the generated current between arms of the dipole antenna or the active area is proportional to the biased field and conductivity of the material. The current surge generates an electromagnetic burst with THz frequency components. Due to the high refractive index of the semiconductor substrate, radiation from the antenna is typically greatest through the substrate, and the THz beam 130 is collected from the back of the PCA chip. Many alternative antenna structures have been demonstrated in addition to the simple dipole of FIG. 2, each with unique properties.
The same PCA can be configured as a receiver when it is electrically connected to a lock-in amplifier (FIG. 1). In this case no DC bias is used. Rather, the received THz signal biases the PCS causing current to flow across the gap. This is sampled using a replica of the transmitter pump pulse, such that a signal proportional to the received THz signal is detected by the lock-in. The delay line (FIG. 1) shifts the position of the sampling THz pulse and provides discrete time samples proportional to the amplitude of the THz electric field. A real-time THz frequency spectrum is obtained by taking a Fourier transform of the sampled THz pulse.
The present disclosure addressing the above shortcomings and provides approaches that can realize improvements in power and efficiency of photoconductive switches. Such improvements will be essential in making THz measurement practical.